Mutations in mitochondrial (mt)DNA are associated with a wide range of human diseases including premature aging, myopathies, neurodegenerative diseases, diabetes, cancer and infertility. In light of the fact that many of these disorders are dependent on the heteroplasmic state of the mtDNA and associated threshold effects, it is difficult to provide accurate genetic counseling based on preimplantation or prenatal genetic diagnoses. At present, there are no cures for mitochondrial disorders and available treatments only improve symptoms and slow disease progression. The main goal of this proposal is to generate important new insights concerning feasibility, efficacy and safety of novel reproductive options designed to minimize the occurrence of mtDNA- defects in a clinically relevant nonhuman primate model. Our main hypothesis is that mtDNA can be efficiently replaced by a novel approach, i.e., spindle transfer (ST) in mature metaphase II (MII) oocytes without interfering with subsequent nucleo-mtDNA compatibility and developmental competence. Our preliminary studies demonstrate the feasibility and efficacy of this approach in the rhesus monkey. We believe that reconstructed oocytes produced after spindle transfer will be nearly homoplasmic, capable of supporting normal fertilization and competent for full term development. To achieve this goal we propose the following specific aims: 1. Develop efficient mtDNA replacement approaches in rhesus monkey oocytes. Our working hypothesis is that unfertilized, mature MII-arrested oocytes are the most optimal stage for mtDNA interventions. We will explore several ST procedures and evaluate their impact on spindle integrity, fertilization and in vitro embryo development. We will also evaluate feasibility and efficacy of ST with cryopreserved oocytes. 2. Investigate developmental potential of reconstructed oocytes and assess mtDNA heteroplasmy and epigenetic profiles in ST offspring. Initially, we propose to derive embryonic stem (ES) cells and to examine karyotype, pluripotency, imprinting and mtDNA heteroplasmy in vitro. Next, we will evaluate the potential of reconstructed embryos to establish pregnancies and produce normal infants, the ultimate test before applications in humans. We will also investigate segregation of mtDNA variants in various tissues and organs of ST offspring. 3. Study growth and development of monkeys produced by ST and examine mtDNA transmission in the ST female germline. Our assumption is that mtDNA replacement therapy will not affect normal postnatal growth and development of ST offspring. We will study experimentally created monkeys from birth to age 5 in comparison to control animals. Due to the genetic bottleneck in the female germline, some offspring of heteroplasmic females may inherit a significant portion of mtDNA from the nuclear donor. Therefore, we will investigate mtDNA heteroplasmy in oocytes collected from ST females by creating embryos and ES cells. PUBLIC HEALTH RELEVANCE: Mutations in mitochondrial DNA are associated with a wide range of human diseases, however, there are no cures for mitochondrial disorders and available treatments only improve symptoms and slow disease progression. Mitochondrial replacement in eggs prior to fertilization offers a potentially efficient and ethically acceptable strategy to avoid transmission of the vast array of mitochondrial disorders in affected families to children. In this study, we will explore feasibility, efficiency and safety of this novel mitochondrial gene replacement therapy in a clinically relevant nonhuman primate model.